Tech Tips

Good to Know About Superglue

The cyanoacrylate glue, popularly referred to as superglue is one of
those additions to the workbench which brought a small technological
revolution in modelling. Not only did it allow to bond different
materials such as metal to plastic, but also opened up the way for
entirely new modelling materials such as resin. Used on plastic, it
provides non-shrinking, non-destructive seams that are just great
for sanding. Applied as filler, it takes scribing unlike any other.

Since there seems to be a degree of confusion about how
cayanoacrylate glue works and what it is best used for, I have made
some research, the results of which are presented here. The
information contained below comes from interviews I made at the
the Department of Fibre and Polymer Technology at the Royal
Institute of Technology in Stockholm and from a variety of sources on the Internet.

What is
Superglue?

The correct chemical designation for the CA glue is ethyl
2-cyanoacrylate. An acronym ECA is also used in chemistry. There are
also numerous trade names, including superglue (of course),
permabond, pro grip, black max, krazy glue, cyanolite, superbonder
and so on.

A common source of confusion is mixing up CA glue with Locktite
thread locking glue. The main reason for this seems to be
historical. The Loctite brand that initially produced the thread
locking glue became for many users synonymous with that type of
glue. Nowadays, Loctite has an entire variety of glue products in
its range, CA glue among them. The main difference between the two
types is in the medium activating the bonding reaction. In the
thread locking glue, polymerisation starts in the absence of oxygene
(air), while CA bonds with the aid of humidity.

How it Works

The cyanoacrylate glue hardens very quickly when trapped between
two surfaces. The reaction is caused by the condensed water vapour
on the surfaces (namely the hydroxyl ions
in water). The water comes from the surrounding air, so
obviously the air humidity is a factor that may affect bonding
capabilities, or cause them to differ from application to
application.

The curing reaction starts at the surface of the bonded
material and develops towards the centre of the bond. Because of
this, thick seams or large blobs of glue may harden less
satisfactorily than surface-to-surface bonds with good fit. In a
thick blob of glue, a polymerisation reaction may stop before it
reaches the centre of the blob. A rule of thumb is that seams
thicker than 0,25 mm should be avoided. Thick seams will also take
longer time to cure.

The described relation between seam width and curing time can be
used to advantage: a thick superglue-filled seam will allow
adjustment of the parts, but will bond instantly and definitely when
they are pressed together, so that the gap decreases below 0,25 mm.
Pressing the parts harder against each other will make the glue cure
instantly.

CA applications

The best use for CA glues is undoubtedly attaching small details,
where small amount of glue would cater for thorough polymerisation
and advantage can be taken of the extremely fast bonding time.

CA glue will provide strong bonds on a wide variety of materials.
The shearing and pulling resistance are very good. However, it
should not be used on glass or on parts that are exposed to water.

Curing time and slow-setting inhibitors

The hardening reaction can be described like this. The
cyanoacrylate is a polymer which contains its own hardener compound.
However, a weak acid is added acting as an inhibitor, preventing the
reaction and “holding apart” the molecules which accounts for the
liquid consistency of the compound. When exposed to water, the acid
is dissolved. It triggers a chain reaction and the compound cures to
the solid state.

Manufacturers use the inhibitor to control the curing time of the
glue. Slow-setting superglues have a larger proportion of inhibiting
acid in the basic mixture.

Accelerators

Besides water, cyanoacrylate polymerises also in presence of
alcohol and basic compounds (including weak amines). The latter can
be used to produce a superglue “kicker” – a compound which triggers
quick polymerisation of the glue.

Baking soda is one well-known substance with this effect. If you
apply a layer of superglue to a seam and gently pour baking soda
over it, the glue will cure very quickly. It makes for most
effective filler for smaller jobs, and the baking soda results in a
slightly rough surface which is good for sanding.

The great advantage of using superglue as filler is the total
absence of shrinking which plagues most solvent-based fillers on the
market.

There are also commercially available liquid accelerators, but as
baking soda has the same effect, I personally prefer it over another
harmful chemical in my workshop.

Like water, the accelerator also affects the reaction through
surface contact, so it will be much less effective on thick layers
of glue. When filling larger recesses with superglue, it is
therefore advisable to build up the volume in several thin layers
rather than applying a large volume of glue at once.

With these precautions (i.e. working with small amounts at a
time), CA can also be used for moulding smaller detail parts, which
I have tried with success.

A word of health warning

Be mindful that superglue comes with its own set of health
hazards.

The glue has a distinctive, strong, acid odour. Breathing
cyanoacrylate fumes is irritating for your breathing organs. For
some individuals, repeated or extended exposure to fumes may prompt
chronic allergic reaction. In dry air (less than 50% humidity),
fumes may be also be irritating to eyes, stimulating tears.

In contact with the skin, the primary risk with the CA is bonding
fingers or other body parts together. This goes also for eyelids, no
remember to never ever poke your hands into the eyes while working
with superglue!

In skin or eye contact, CA is deemed to be non-toxic, so don’t
panic, assess the situation, and seek medical help if necessary.
Never try to tear the apart the bonded body parts!

It is also ascertained that CA cannot trigger allergic reactions
through skin contact.

The wisdom of the above is that safety glasses and breathing
masks are best worn when working with these glues. A good
ventilation is also a must.

n

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